Pertinent areas of the Classification Manual concerned with this type of invention are, among others, Class 208 subclass 310, and Class 585 subclasses 820 and 701. In Gray, Jr. et al, U.S. Pat. No. 4,476,345, an invention is disclosed in which a portion of one of the product streams in a total isomerization system is used to wash a recycle gas stream to improve the quality of the products. The molecular sieve adsorbent is selected from naturally occurring or synthetically produced three-dimensional crystalline zeolitic aluminosilicates which selectively, on the basis of molecular size, absorb normal paraffins from the isomerized product from branched chain and/or cyclic paraffins. These molecular sieves have pore diameters of about 5 Angstroms and are exemplified by a Zeolite A, which exhibits pore diameters ranging from about 3 to about 5 Angstroms. This recycle wash contains only a small amount of non-normals which enables lowering of the undesired residuals. This process recycles directly in the vapor phase in combination with the feedstock, a mixture of normals and non-normals purged from each separatory bed to another separatory bed in the system. Exemplary of this invention is the description of the drawings set forth at Column 6 lines 1 through 14.
The Gray et al disclosure is an improvement upon a Total Isomerization Process as taught in Holcombe, U.S. Pat. No. 4,210,771. This is a process for the virtually complete isomerization of normal paraffin hydrocarbons in a feedstream consisting essentially of mixed normal and branched hydrocarbons, where the feedstream is passed first through an isomerization reactor and the products therefrom passed to an adsorption section which separates normal from branched paraffins to form an isomerate having di- and mono-branched paraffins and a recycle stream of nearly pure normal paraffins. This basic process is sometimes referred to as a Total Isomerizaiton Process (TIP). The instant invention is a basic improvement upon said process.
The zeolitic molecular sieve employed in former TIP processes is any type of adsorption bed capable of selectively absorbing normal paraffins employing the molecular size and configuration of the normal paraffin molecules as a selection criteria. Particularly suitable zeolites of this type are Zeolite A and calcium exchanged Zeolite 5A. Other naturally occurring zeolite molecular sieves include chabazite and erionite. The flow scheme of the TIP process, as exemplified in Holcombe, is herein incorporated by reference as a teaching of how to operate multiple zeolite molecular sieves to achieve proper adsorption-fill and desorption purge in an operable process. In the drawing of Holcombe, the adsorption bed systems, 44, 46, 48 and 50, are comprised of calcium 5A Zeolite in the form of 1/16" cylindrical pellets. Branched paraffins flow through the adsorption bed while normal paraffins are adsorbed. After purge of the normal paraffins from the zeolitic molecular sieve, a recycle stream is formed of normal paraffins and recycle hydrogen where the recycle stream is mixed with incoming fresh feed before charge to the patentee's isomerization zone.
A second Holcombe patent, U.S. Pat. No. 4,176,053, discuses a normal paraffin-isomerization separation process. By this technique, normal paraffins are isolated from a feedstock mixture comprising normal and branched paraffins at super atmospheric pressure using an adsorption system comprising at least four fixed adsorbent beds containing a calcium 5A Angstrom molecular sieve. A stream is formed comprising vapor from void space purging of the adsorbent and feedstock containing isoparaffins and normal paraffins. The molecular sieve employed to separate normal paraffins from said stream is one which adsorbs only normal paraffins from a mixture of branched, cyclic and normal hydrocartons in order to segregate the normal paraffins from said mixture.
These patents teach that it is most advantageous to recycle normal paraffins to exhaustion. These patents also teach that the isomerate will have a certain quantity of mono-methyl-branched hydrocarbons derived from the isomerization of normal paraffins in an upstream isomerization zone. In contrast, applicants have discovered a new and more efficient Total Isomerization Process whereby both normal paraffins and mono-methyl-branched chain hydrocarbons are recycled to increase and optimize the quantity of di-branched paraffins in the isomerate. Using the specific molecular sieves of the aforementioned patents, mono-methyl-branched hydrocarbons such as methylpentanes are untrapped and are present in the product. On the other hand, by use of the select molecular sieves of the instant process, mono-methyl-branched paraffins such as methylpentanes, are absorbed in the sieve and, after desorption, along with normal paraffins, are recycled to the feedstream or to the isomerization zone, wherein normal paraffins and mono-methyl-branched hydrocarbons are isomerized to multimethyl-branched paraffins such as dimethylbutane. This process increases the degree of branching existent within the isomerate, which increases the octane number. For example, in the instant process, increased concentrations of dimethylbutanes would result from C.sub.6 feeds, relative to the prior art. In summary, both the select molecular sieve of this invention and the calcium-5A sieve preferred by the patentees adsorb normal paraffins. The select molecular sieve of this invention adsorbs mono-methyl-branched paraffins while the calcium 5A sieve does not perform such an adsorption. Finally, neither the sieve of this invention nor the calcium 5A sieve absorbs dimethyl-branched paraffins, such as dimethylbutane, which is desired to be present in the isomerate of a Total Isomerization Process.